Genetic Background of Prop1df Mutants Provides Remarkable Protection Against Hypothyroidism-Induced Hearing

Hypothyroidism is a cause of genetic and environ-mentally induced deafness. The sensitivity of cochlear development and function to thyroid hormone (TH) mandates understanding TH action in this sensory organ. Prop1df and Pou1f1dw mutant mice carry muta-tions in different pituitary transcription factors, each resulting in pituitary thyrotropin deficiency. Despite the same lack of detectable serum TH, these mutants have very different hearing abilities: Prop1df mutants are mildly affected, while Pou1f1dw mutants are completely deaf. Genetic studies show that this difference is attributable to the genetic backgrounds. Using embryo transfer, we discovered that factors intrinsic to the fetus are the major contributor to thi